The integrated circuit industry demands chemicals with higher purity and lower levels of particle contamination in order to produce very large-scale, integrated (VLSI) circuits. While current techniques are utilized to improve the quality of VLSI chemicals, there is a requirement that delivery of VLSI chemicals to the point of use and during use be controlled to eliminate contamination. Thus in the past there has been a substantial effort expended in the area of packaging in which "clean room" techniques and "double bag" techniques are utilized to reduce the contamination of chemicals when they are employed to clean or modify the surface of a semiconductor wafer.
Traditionally, the semiconductor industry has utilized batch processing in which "boats" containing about twenty-five silicon wafers are processed stepwise and frequently are moved from process area to process area manually. To reduce human error, as well as to reduce the level of human-contributed particulate contamination, the semiconductor industry is moving toward continuous processes. At the same time, with the steadily shrinking geometries of the devices on the wafer, uniform processing of the wafers is becoming more important. With batch processing, it is not always possible to react each of the wafers in the boat or different regions across the wafer in the same manner. Thus it is important to develop an integrated continuous system which can be used at the wafer cleaning step, at the resist developing step and at the etching step.
Not only is uniform treatment from wafer to wafer growing in importance, but also precise control of the extent of the reaction is becoming critical. For instance, precise development of the photoresist is important to insure that patterns formed are of exactly the desired size. This is becoming increasingly important for such devices as the 256K memory chips presently in production.
Of particular concern in the reduction of contamination is wafer cleaning. As indicated in an article by Aaron D. Weiss, in Semiconductor International, Volume 82, April 1984, there are basically four types of wafer cleaning, each with its own problems. The first is brush scrubbing, including high pressure scrubbing; the second is sonic cleaning, utilizing ultrasonic techniques; the third is chemical cleaning in a bath; and the fourth utilizes a centrifugal spray.
As indicated in this article, shrinking of device geometries has been the driving force in the semiconductor industry. A primary concern in the fabrication of VLSI devices is the level of contamination the wafers are exposed to and removal of such contamination from the surface of the wafers before further processing. Each of the different wafer cleaning techniques described above has advantages for removing a specific type of contamination. For example, wet chemical cleaning is efficient in removing alkaline metal ions that may be either physically or chemically attached to the wafer surface. These techniques are highly specific to the type of contaminant and are also subject to the condition of the wafer surface. It will be appreciated that cleaning of wafers is done after sawing, lapping and polishing; before coating with resist, before diffusion, and after dicing. The types of contaminants range from organics encountered by handling or processing to heavy metal ions and particulates. Contaminants may be visible or invisible and may either chemically or physically attached to the wafer surface. Generally, any contaminants on the wafer can decrease the device performance and, in some cases, completely destroy the device. For instance, contaminants left on the wafer can cause pin holes and poor resist adhesion. Particulates left on the wafer can cause device defects which are exemplified by shifts in electrical parameters and complete device failure.
With respect to brush scrubbing, the bristles of the brush do not actually come into contact with the wafer surface because of the hydrophilic nature of the brush material. Thus there is usually a film of the scrubbing solution between the brush bristles and the wafer surface. Secondly, the hydrophilic character of the bristles helps sweep off suspended contaminants from the wafer surfaces that are hydrophobic. Wafers that have a surface that is hydrophilic are more difficult to scrub because contaminants suspended in the scrubbing solution can precipitate onto the wafer topography. Scrubbing, while it will remove particulates, will not remove contaminants unless they are soluble in the scrubbing solution, which is most often water based. Furthermore, contaminants that are chemically attached to the wafer surface cannot be removed by scrubbing alone. This requires the use of a chemical agent or solution to release the contaminant from the wafer surface. Chemical cleaning can be used in conjunction with scrubbing. However, as device geometries shrink, there is concern that the bristles of the brushes may not effectively remove contaminants. In order to eliminate this problem, high pressure scrubbing with a jet of fluid swept across the wafer surface at pressure of from 300-4000 psi is useful to get into topography which the bristles cannot touch. However, the major problem with high pressure sprays is static electricity which damages the delicate devices which are being manufactured into the wafer.
Ultrasonic cleaning, which forms bubbles from cavitation, is utilized in order to scrub wafer surfaces, while megasonic scrubbing utilizes high pressure waves set up in the solution rather than implosion of bubbles. Note, the frequency of the megasonic energy is too high for the creation of bubbles. Regardless of the utilization of ultrasonic cleaning, wafer handling is still a problem in that the removal of the wafer from the sonic bath into the atmosphere and then to another place for processing can engender the pickup of contaminants. Thus a much-stated goal in the semiconductor industry is removal of people from the clean room in order to eliminate one of the major sources of contaminants. To do this, processing equipment must have automatic wafer handling mechanisms. These mechanisms, however, can be a source of particulate generation. Therefore, an important aspect of the automated wafer cleaning equipment is the wafer handling mechanism which must not generate particulates in and of itself. Furthermore, it must not break wafers. Broken wafers can be a catastrophe in a scrubber, which results in the generation of large quantities of silicon particulates.
With respect to chemical cleaning, various dip and dunk techniques are utilized which result in contaminated wafers during transfer to and from the bath. An alternative to the immersion type system is the centrifugal spray cleaning mentioned above in which a centrifugal spray cleaning unit sprays fluids onto the wafers such that each wafer is continuously exposed to fresh cleaning or rinsing solution. This helps prevent recontamination of the wafers by dirty solutions. The advantage of this type of chemical cleaning is that the wafers are housed in a closed environment throughout the entire process.
A drawback of the sonic as well as the dip and dunk methods is that the substrates must be withdrawn from the tank through the liquid surface which is well known to collect particulates. Thus, the particulates are redeposited on the clean substrate when it is withdrawn from the liquid. Even with a spray, the surface of the liquid droplet can sweep particulate contamination from the environment and deposit it on the substrate surface.
One major drawback with respect to the spraying systems is that the uniformity of the chemical reaction produced by the sprayed material cannot be carefully controlled. Moreover, the distribution of chemicals on the wafer surface is not uniform. Additionally and more importantly, the chemical reaction cannot be measured by optical methods when using a spray or mist. Therefore, careful control of the chemical reaction on the wafer surface cannot easily be accomplished when utilizing spray techniques.
Finally, wafer drying is a critical step after the cleaning technique. The wafers must be dried in such a way as to prevent recontamination. The technique used for drying depends in large measure on the type of wafer surface, either hydrophilic or hydrophobic. In general, hydrophobic surfaces are easier to dry than hydrophilic surfaces. A common method utilized for drying is spin drying. For a hydrophilic surface, the spin speed should be carefully controlled to prevent an aerosol from forming which can recontaminate the wafer. Spinning should start out at low rpm until all but a thin film of liquid is left on the wafer surface, whereupon the spin speed can be increased. Drying hydrophobic wafers is different in that water droplets form and roll off as the wafer is spun. However, at the center of the wafer the van der Waals forces prevent the droplets from rolling outward. Thus a flow of nitrogen directed at the center of the wafer is needed to move the droplets. Nitrogen is generally not used for hydrophilic wafers because it will cause an aerosol to form.
In all of the prior art devices thus described, it is possible during the processing that the surface of the wafer can be contaminated because there are times at which unreacted active chemicals on the wafer surface are exposed to air. This is due primarily to the lack of fluid layer between the ambient and the wafer surface. Thus no matter how pure the chemicals used in the processing of the wafer, should any of these active areas be exposed to the atmosphere, contamination will result.
U.S. patents dealing with machine cleaning of semiconductor wafers include U.S. Pat. Nos. 3,760,822; 4,015,615; and 3,990,462. An example of the jet/brush scrubber is Solitec Model 1100-SD available from Solitec, Inc. in Mountain View, California.
Two U.S. patents which deal with a method of providing for a chemical reaction which travels across a flat rotating plate are U.S. Pat. No. 4,124,411 and U.S. Pat. No. 4,356,133. Also of note is an article in the Journal of Crystal Growth, Volume 41, No. 2, December 1977, pages 205-215, which describes the rotating disc method for growth of a number of crystals. In none of the apparatus described in the prior art is a predetermined fluid gap and an ever-present fluid proposed between the top of the wafer surface and the fluid guide, which is the subject of the present invention.
Attention is also drawn to Japanese Patent No. GO6 84-040358/07 J5 9003430-A issued to Fujitsu Ltd. in which photoresist is apparently coated on two rotating opposed base plates. Here there is no fluid guide or fluid dispensed through a fluid guide nor is there any monitoring of mixing reactions through a fluid guide.
While the processing of VLSI wafers has been previously discussed, cleaning and other types of processing of other types of substrates in which contamination is a problem include masks, optical windows and plates, and printed circuit boards.